1836 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
wet solids were separated by decantation and calcined in a
furnace at 700 °C yielding a MREO product. The resulting
MREO purity was estimated to be 99%. The composition
of the MREO product is shown in Table 6.
Preliminary Technoeconomic Analysis (TEA)
The preliminary TEA was performed to compare the eco-
nomic feasibility of the hydrometallurgical process (phos-
phoric acid (PA,54% P2O5)-rare earth oxide (REO)) with
that of the thermal process (PA-P4 (elemental phosphorus)-
REO). In the TEA, mass and energy balances were used to
estimate both the operating and capital costs of the process-
ing plant, as well as the levelized value of the REE product.
The technoeconomic analysis considered both the capital
and operating costs to build a new facility against an “as-
is” operation where the phosphoric acid sludge is sold as
a low-grade fertilizer. Mass and energy balances were used
to estimate the size of equipment and the operating costs
associated with each unit operation. Currently, the Mosaic
Company produces a mass of 453,600 tons of solid sludge
per year, which serves as the baseline for the mass and energy
balances in these proposed processes. The recovered PA,
P4, and processed REO solids (97.4% purity) are 340,000
metric tons/yr, 2,570 metric ton/yr and 38.0 metric tons/
year respectively, in the PA-P4-REO process.
Capital expenditures (CAPEX) and Operational
expenses (OPEX) of the proposed processes were calcu-
lated based on standard assumptions. The operating cost of
PA, P4, and REO is determined to be $0.10/ton, $2,760/
ton and $591.54/kg, respectively. The second scenario of
PA-REO production does not include P4 production.
Without P4 production, the operating cost of REO par-
ticles can be further reduced to $202.65/kg, because the
REO product increases to 109.5 ton/yr. The majority of
the revenue in the PA-REO process comes from phos-
phoric acid (e.g., 54% P2O5, $458/ton), which accounts
for 95.6%. The production of P4 (e.g., $5,721/ton) sig-
nificantly increases the revenue compared to REO (e.g.,
$18,200/ton based on REE basket price). In the PA-P4-
REO process, the majority of the revenue still comes from
PA (82.3%). The total expenses are significantly lower than
the revenues from the two processes. Based on the CAPEX,
OPEX and revenues, we determined the net present value
(NPV), which is the sum of all future cash flows over the
investment’s lifetime, discounted to the present value (i.e.,
capital cost and operating expenses). The calculation of
delta net present value requires the net present value of the
sludge sold as-is for component of low-grade fertilizer (i.e.,
the value is estimated to 10% merchant grade phosphoric
acid). The NPV of PA-REO scenarios is evaluated at $75
million, while that of NPV of PA-P4-REO is $79 mil-
lion for next 10 years after building up a facility. Overall,
through the TEA, we found that both processes are profit-
able and feasible [Jang et al., 2023]. It should be pointed
out that this promising TEA conclusion was based on rela-
tively high prices for phosphoric acid and elemental P and
low value for phosphoric acid sludge. All those numbers are
market-dependent and company-specific.
CONCLUSIONS
A continuous separation of phosphoric acid and solid par-
ticles from phosphoric acid sludge has been realized in lab-
oratory test by using a decanter centrifuge. Simultaneous
recovery of phosphoric acid liquid at ~95% efficiency and
REEs-containing particles at ~90% efficiency in a contin-
uous mode has been achieved in a single pass. Based on
separation via centrifugal acceleration up to 1500 G, the
decanter centrifuge increases the settling rate of micron-
size particles, thus yielding rapid and effective solid/liquid
separation.
This research shows that nearly complete REE leach-
ing recovery is achievable from the phosphoric acid sludge
solids after the materials are roasted. However, the sludge
is also amenable for nitric acid leaching through all hydro-
metallurgical treatments gaining REE recovery of about
90% in multi-stage leaching. The leachates from both the
hydrometallurgical and thermal processes are suitable for
the solvent extraction technology developed by the research
team at PNNL.
The thermal processing demonstrated that after reduc-
tion roasting, the REEs and phosphorus leaching efficien-
cies increased to 98% and 99%, respectively. The roasting
can not only activate REEs by decomposing gypsum and
REE-bearing compounds, but also generate white phos-
phorus which is more valuable than phosphoric acid recov-
ered through leaching the sludge. The products SO2 and/
or CaS from roasting can be used to produce sulfuric acid,
which can be recycled back to phosphoric acid produc-
tion. So, the thermal processing has a potential to recover
and recycle more values. However, obviously, more energy
consumption is one of its shortcomings compared with the
hydrometallurgical process.
Table 6. Composition of the MREO Product
Element Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Total
Wt% 34 2.2 11 3.7 23 7.2 1.6 9.2 .92 4.3 .53 1.0 .007 .26 .004 99.0
wet solids were separated by decantation and calcined in a
furnace at 700 °C yielding a MREO product. The resulting
MREO purity was estimated to be 99%. The composition
of the MREO product is shown in Table 6.
Preliminary Technoeconomic Analysis (TEA)
The preliminary TEA was performed to compare the eco-
nomic feasibility of the hydrometallurgical process (phos-
phoric acid (PA,54% P2O5)-rare earth oxide (REO)) with
that of the thermal process (PA-P4 (elemental phosphorus)-
REO). In the TEA, mass and energy balances were used to
estimate both the operating and capital costs of the process-
ing plant, as well as the levelized value of the REE product.
The technoeconomic analysis considered both the capital
and operating costs to build a new facility against an “as-
is” operation where the phosphoric acid sludge is sold as
a low-grade fertilizer. Mass and energy balances were used
to estimate the size of equipment and the operating costs
associated with each unit operation. Currently, the Mosaic
Company produces a mass of 453,600 tons of solid sludge
per year, which serves as the baseline for the mass and energy
balances in these proposed processes. The recovered PA,
P4, and processed REO solids (97.4% purity) are 340,000
metric tons/yr, 2,570 metric ton/yr and 38.0 metric tons/
year respectively, in the PA-P4-REO process.
Capital expenditures (CAPEX) and Operational
expenses (OPEX) of the proposed processes were calcu-
lated based on standard assumptions. The operating cost of
PA, P4, and REO is determined to be $0.10/ton, $2,760/
ton and $591.54/kg, respectively. The second scenario of
PA-REO production does not include P4 production.
Without P4 production, the operating cost of REO par-
ticles can be further reduced to $202.65/kg, because the
REO product increases to 109.5 ton/yr. The majority of
the revenue in the PA-REO process comes from phos-
phoric acid (e.g., 54% P2O5, $458/ton), which accounts
for 95.6%. The production of P4 (e.g., $5,721/ton) sig-
nificantly increases the revenue compared to REO (e.g.,
$18,200/ton based on REE basket price). In the PA-P4-
REO process, the majority of the revenue still comes from
PA (82.3%). The total expenses are significantly lower than
the revenues from the two processes. Based on the CAPEX,
OPEX and revenues, we determined the net present value
(NPV), which is the sum of all future cash flows over the
investment’s lifetime, discounted to the present value (i.e.,
capital cost and operating expenses). The calculation of
delta net present value requires the net present value of the
sludge sold as-is for component of low-grade fertilizer (i.e.,
the value is estimated to 10% merchant grade phosphoric
acid). The NPV of PA-REO scenarios is evaluated at $75
million, while that of NPV of PA-P4-REO is $79 mil-
lion for next 10 years after building up a facility. Overall,
through the TEA, we found that both processes are profit-
able and feasible [Jang et al., 2023]. It should be pointed
out that this promising TEA conclusion was based on rela-
tively high prices for phosphoric acid and elemental P and
low value for phosphoric acid sludge. All those numbers are
market-dependent and company-specific.
CONCLUSIONS
A continuous separation of phosphoric acid and solid par-
ticles from phosphoric acid sludge has been realized in lab-
oratory test by using a decanter centrifuge. Simultaneous
recovery of phosphoric acid liquid at ~95% efficiency and
REEs-containing particles at ~90% efficiency in a contin-
uous mode has been achieved in a single pass. Based on
separation via centrifugal acceleration up to 1500 G, the
decanter centrifuge increases the settling rate of micron-
size particles, thus yielding rapid and effective solid/liquid
separation.
This research shows that nearly complete REE leach-
ing recovery is achievable from the phosphoric acid sludge
solids after the materials are roasted. However, the sludge
is also amenable for nitric acid leaching through all hydro-
metallurgical treatments gaining REE recovery of about
90% in multi-stage leaching. The leachates from both the
hydrometallurgical and thermal processes are suitable for
the solvent extraction technology developed by the research
team at PNNL.
The thermal processing demonstrated that after reduc-
tion roasting, the REEs and phosphorus leaching efficien-
cies increased to 98% and 99%, respectively. The roasting
can not only activate REEs by decomposing gypsum and
REE-bearing compounds, but also generate white phos-
phorus which is more valuable than phosphoric acid recov-
ered through leaching the sludge. The products SO2 and/
or CaS from roasting can be used to produce sulfuric acid,
which can be recycled back to phosphoric acid produc-
tion. So, the thermal processing has a potential to recover
and recycle more values. However, obviously, more energy
consumption is one of its shortcomings compared with the
hydrometallurgical process.
Table 6. Composition of the MREO Product
Element Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Total
Wt% 34 2.2 11 3.7 23 7.2 1.6 9.2 .92 4.3 .53 1.0 .007 .26 .004 99.0